Field of the Invention
The present invention concerns the technical field of magnetic resonance imaging, in particular a method for acquiring a magnetic field inhomogeneity value and a distortion correction method for a magnetic resonance imaging system.
Description of the Prior Art
Magnetic resonance imaging (MRI) is a technology in which the phenomenon of magnetic resonance is utilized for the purpose of imaging. The basic principles of magnetic resonance are as follows. When an atomic nucleus contains a single proton, as is the case with the nuclei of the hydrogen atoms that are present throughout the human body, this proton exhibits spin motion and resembles a small magnet. Moreover, the spin axes of these small magnets lack a definite pattern, and if an external magnetic field is applied, the small magnets will be rearranged according to the magnetic force lines of the external field; specifically, they will line up in two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis; the atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field such that their spin axes deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance—this is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei have deviated from the positive or negative longitudinal axis, the atomic nuclei have a transverse magnetization component.
Once emission of the RF pulse has ended, the excited atomic nuclei emit an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves, such that their phase and energy level both return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
Echo Planar Imaging (EPI) is a fast magnetic resonance (MR) imaging method which exhibits geometric distortion. To correct this geometric distortion, a number of distortion correction methods have already been presented in MRI systems in the prior art. These methods are moreover already widely used for distortion correction in (specifically) echo planar imaging and Blood Oxygen Level Dependent Functional Magnetic Resonance Imaging (BOLD fMRI). However, these methods depend on pre-obtained magnetic field charts or point spread functions. Specifically, in applications having long measurement times such as BOLD fMRI or Diffusion Tensor Imaging (DTI), the cost of the time taken to measure a magnetic field chart or point spread function is so small as to be not worth mentioning, but in the case of applications having short measurement times such as Diffusion Weighted Imaging (DWI), additional time cannot be ignored. Specifically, diffusion weighted imaging is based on the method of Echo Planar Imaging (EPI) (a fast MRI method).